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the parent
material from which soil forms. There are
soils that stand this little lecture on its head,
for soils are as varied as the rocks, climate,
topography, organisms, and length of time
that create them.
Bacteria-and rain and lightning-pull
nitrogen from the air. Plants take root, suck
up nitrogen and the minerals leached from
rock, throw seeds, and die. Worms, ants,
gophers move in and rearrange the soil,
opening it up and giving it air. Multiplying
beyond count, microbes help release nutri
ents from dead plants for use by live ones by
decomposing organic matter into humus, a
dark adhesive embracing clay particles, giv
ing topsoil the feel of bread crumbs and the
function of a sponge. Roots can now more
easily get water and grow. A soil evolves.
Except for mucks like the Sacramento
Delta, most soils have little humus, maybe 6
percent under prairie and almost none in the
deserts. But humus is far more important
than its proportion in soils indicates. By
Smanuring
or plowing
under
a cover
crop like
clover, a farmer can return nutrients to the
soil for his next rotation of corn.
P LANTS ARE NOURISHED by inor
ganic minerals, so the corn doesn't care
if nitrogen is converted by bacteria from
manure or scattered from a bag of com
mercial inorganic fertilizer, but the farmer
may see the difference in his cash flow.
About half our 8.6-billion-dollar fertilizer
bill is for nitrogen, most of it in the form of
ammonia made from air and natural gas,
making farmers all the more vulnerable to
the volatile prices and politics of oil and gas.
More important, humus helps topsoil
hold water against a dry spell, and by ab
sorbing runoff, it slows erosion. But left un
protected on a hillslope, topsoil gradually
gives up its organic glue to a thin sheet of
moving water. "As slopes erode,"
Klaus
Flach, an SCS scientist, told me, "you get
more runoff and less water infiltrating the
soil. Out in those areas of the country where
water is critical, the crops get starved. We
really saw it in the 1983 drought."
The deserts are fertile because there is lit
tle rain to leach away the mineral nutrients.
That's why Arizona's Salt River Valley
Bloomed when it was irrigated, and why
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